1. Field of the Invention
The invention relates to a load abnormality detecting system and method for detecting the burnout and short circuit of a load.
2. Description of the Related Art
The construction of a conventional load abnormality detecting system 30 is shown in FIG. 2. The conventional load abnormality detecting system 30 has a current sensor 1 that causes a signal current Is corresponding to an amount of a load current IL flowing through a load 2 to be generated and outputs the signal current Is. A relationship of signal current Is=load current IL/K (K: constant) holds between the signal current Is and the load current IL.
A resistor R is disposed between a wire, in which the signal current Is flows, and a ground potential. Accordingly, a voltage Vs corresponding to an amount of the signal current Is is generated at a connection point of the wire to the resistor R. Here, the voltage Vs is a product of the signal current Is and a resistance value r, Is×r, if r denotes the resistance value of the resistor R.
The voltage Vs is converted into a digital signal in an AD converter 3, and the digital signal is inputted to one input portion of a judging device 4. A burnout reference value T1 or a short-circuit reference value T2 is inputted to the other input portion of the judging device 4.
If the burnout of the load 2 wants to be checked, the burnout reference value T1 is caused to be inputted to the other input portion of the judging device 4 by controlling a selecting means 5. The selecting means 5 is for switching the selection by means of an electrical changeover switch or a software control. The burnout of the load 2 is judged by comparing the inputted digital signal and the burnout reference value T1 in the judging device 4.
On the contrary, if the short circuit of the load 2 wants to be checked, the short-circuit reference value T2 is caused to be inputted to the other input portion of the judging device 4 by controlling the selecting means 5. The short circuit of the load 2 is judged by comparing the inputted digital signal and the short-circuit reference value T2 in the judging device 4.
Here, the AD converter 3, the judging device 4, the selecting means 5 and the like are incorporated into a microcomputer 20. In other words, whether or not the load 2 has any abnormality such as burnout or short circuit can be judged in the microcomputer 20.
Technology concerning the current sensor 1 is disclosed in Japanese Patent Office's Standard Technology Collection “High-Side Current Power Switch”. Further, technology relating to the present invention (technology of changing a range by switching resistors) is disclosed in Japanese Unexamined Patent Publication No. H08-189845.
In the load abnormality detecting system 30 shown in FIG. 2, the load current IL decreases in the case of burnout in the load 2. Accordingly, the signal current Is decreases and the voltage Vs also decreases as the load current IL decreases as described above. To precisely detect the burnout of the load 2 in the judging device 4, the voltage Vs is desirably larger to improve conversion precision in the AD converter 3. Here, the voltage Vs generated in correspondence with the amount of the signal current Is depends on the resistance value r of the resistor R. Accordingly, it is better to set a large resistance value r of the resistor R in order to precisely judge the burnout in the judging device 4 in the case of the burnout in the load 2.
However, an upper limit is defined for the voltage value inputted to the AD converter 3 incorporated into the microcomputer 20 based on the electrical specification of the AD converter 3 and the like (i.e. an upper limit is defined for the voltage Vs generated in the voltage generator constructed by the resistor R). Accordingly, if the resistance value r of the resistor R is increased only in light of the burnout of the load 2, the voltage Vs having a value in excess of the upper limit may be inputted to the AD converter 3 in the event of the short circuit of the load 2. This is because the load current IL increases in the case of the short circuit in the load 2, and both the signal current Is and the voltage Vs increase as the load current IL increases.
Because of the above, in the actual load abnormality detecting system 30, the resistance value r of the resistor R has been set to a relatively small value so that the voltage value inputted to the AD converter 3 does not exceed the upper limit value of the AD converter 3 also in the event of the short circuit in the load 2.
As described above, the resistance value r of the resistor R needs to be restricted in the conventional load abnormality detecting system 30. Accordingly, in this load abnormality detecting system 30, the resistance value r of the resistor R is set in consideration of accuracy in detecting the short circuit of the load 2 in the event of the short circuit of the load 2 and accuracy in detecting (i.e., judging) the burnout of the load 2 in the event of the burnout of the load 2. Therefore, it has been difficult to improve both accuracies.
Accordingly, an object of the invention is to provide a load abnormality detecting system and method capable of detecting both the short circuit of a load and the burnout of the load with high accuracy even if an upper limit is defined for the value of a voltage to be generated in a voltage generator.